Barroso served as president of the European Commission, the EU's executive arm, from 2004 to 2014 and was prime minister of Portugal from 2002 to 2004.

Goldman Sachs and other U.S. investment banks are seen as particularly vulnerable to Brexit since they rely on the EU's "passporting" regime that allows them to offer services across the bloc while basing most of their staff and operations in the UK. Banks have warned that if their British outposts lose their "passports" they will have to move some employees and business units to alternative bases in the EU.

Goldman Sachs International, which Barroso will chair, is headquartered in London and of its roughly 6,000 staff fewer than 1,000 are based outside Britain.

He was also a signatory to the Lisbon Treaty that revamped the bloc's complex institutions after French and Dutch voters rejected a European constitution.

Barroso will help the firm as it advises clients on dealing with the ensuing "challenging and uncertain economic and market environment," Goldman Sachs International co-chiefs Michael Sherwood and Richard Gnodde said in the statement.